Electron microscopes

ABSTRACT

A combined condenser and objective electric lens for an electron microscope is disclosed. The lens includes two apertured wound pole pieces and an apertured plate between them, the pole pieces and plate having axial apertures. The axial spaces between the plate and pole pieces define condenser and objective gaps, the specimen being positionable in the objective gap. Preferably, the pole pieces are oppositely wound to generate opposing flux which tends to cancel in the plate, avoiding magnetic saturation of the plate.

United States Patent Anderson et al. 1 Dec. 2, 1975 [54l ELECTRON MICROSCOPES 2,418,349 4/1947 Hillier v. 250/396 [75] Inventors: Kenneth Anderson, Bramhall; Kenneth Arthur Brooke-S, Bolton, 318511172 11/1974 Ozasa 250/396 both of England [73] Assignee: Associated Electrical Industries Primary Examiner-Craig E. Church Limited, London, England Attorney, Agent, or FirmWatts, Hoffmann, Fisher & 221 Filed: June 26, 1974 Hcmke 21 Appl. No.: 483,126

[57] ABSTRACT [30] Foreign A li ti P i it D t A combined condenser and objective electric lens for June 28 1973 United Kingdom" 30764/73 an electron microscope is disclosed. The lens includes two apertured wound pole pieces and an apertured 52 US. Cl 250/311- 250/396 Plate between them the P016 Pieces and Plate having 51 Int. Cl. 110m 37/26 axial aperturesaxial spaces between the Plate 58 Field of Search 250/396 397 307 311 and Pole Pieces define d ser and objective gaps, 250/310 1 1 the specimen being positionable in the objective gap. Preferably, the pole pieces are oppositely wound to [56] References Cited generate opposing flux which tends to cancel in the UNITED STATES PATENTS plate, avoiding'magnetic saturation of the plate.

2,323,328 7/1943 Hillier .1 250/396 13 Claims, 3 Drawing Figures U..S.. Patent Dec. 2, 1975 shw 1 of 2 3,924,126

mo El US. Patent Dec. 2, 1975 Sheet 2 of2 3,924,126

%\W wk [a w ELECTRON MICROSCOPES BACKGROUND OF THE INVENTION are accelerated by an anode, and are focused onto the.

specimen by a condenser lens. The specimen is usually, situated within the magnetic field of an objective lens which forms, with the electrons transmitted through the specimen, a magnified image of the specimen. This image is then projected by one or more projector lenses I onto a fluorescent screen. The intensity of illumination of the specimen (i.e., the electron current density) is controlled by the condenser lens, being at a maximum when the condenser lens is set so as to form a focused image of the illumination source on the specimen. The current density can be reduced by either reducing the gun brightness, reducing the size of the condenser aperture, or by spreading the illumination over a larger area by changing the focus of the condenser lens, this latter being the normal method for frequent changes.

The minimum probe size (i.e., size of the area illuminated at the specimen) occurs when the illumination source is imaged onto the specimen by the condenser lens, and under these conditions the probe size is given by the product of initial source size and condenser lens magnification. I

Now in the design of the microscope, it is usually necessary to leave sufficient space between the condenser and objective lenses to accommodate an electrical stage for controlling the beam position and direction, and a specimen stage and airlock mechanism for exchanging specimens. Thus the working distance of the condenser lens (i.e., the distance from condenser lens to specimen) is usually comparable to the 'distance from condenser lens to the source so that 'the condenser lens magnification is approximately unity, giving a probe size of the same order as the source size, e.g., of the order of 50 microns for a thermionic hairpin source. I

An electron microscope with a single condenser lens as described suffers from the serious disadvantage that, for a reasonable specimen current density, the total current onto the specimen is large because of the large probe size. This can result in serious heating problems at the specimen and marked deterioration in image quality. It is for this reason that early in the development of high resolution instruments, a second condenser lens was added to the illuminating system. In such a two-condenser system, the first condenser lens, being a strong lens, de-magnifies the source to an image size typically of the order of 2 microns and this is focused onto the specimen by the second condenser lens with a magnification of approximately unity, to give a probe size of the order of 2 microns.

With the more recent introduction of -X-ray analytical techniques to the transmission electron microscope, in which X-rays produced from the specimen by the electron beam are detected and measured, it has become very desirable to produce much smaller probe sizes, down to 0.1 micron or less, in order to improve the spatial resolution of the X-ray detector systems and at the same time it is necessary to maximise the current into the probe to improve sensitivity. In addition, considera'ble attention has to be paid to the mechanical design to allow the X-ray detectors to be placed inclose proximity to the specimen and in a direction of strong X-ray'yield so as not to lose sensitivity.

The requirement of high current probes at the submicron level cannot be met simply by the additionof a further stage of de-magnification before the final condens er lens. In addition to'the undesirable increase in column height this would entail, and the consequent worsening of mechanical stability, a more fundamental objection arises from consideration of theaberration of the final probeforming condenser lens. Spherical aberration in this lens causes electrons which pass through its periphery to be brought to a different axial focus from the electrons which pass close to the lens centre, giving rise to the well-known spherical aberration circle of confusion.

7 It can be shown that, in order to reduce the size of the aberration disc to a value compatible with the required I 0.1 micronprobe size, the magnification factor of the final condenser lens must be made small compared with unity. Hence for useful probe-forming lenses in the submicron region a small working distance is essential.

One way in which the required small working dis-, tance has been achieved (Cooke and Openshaw, Proc. 4thNat. Conf. on Electron Microprobe Analysis, Pasadena, Calif, 1969) is by the used an ironfree miniature lens, the small physical size of such a lens enabling it to be located very close to the objective lens without obstructing the passage of X-rays emitted from the specimen. However, the small size of the lens also leads to high current densities within the windings and consequent water cooling problems. An even greater problem with the miniature lens is. that the field symmetry is=determinedby the symmetry of the windings and not, as in an iron lens, by the much more easily obtained symmetryof the pole piece machining. Thus in the miniature lens great attention has to be paid to uniformity of windings, in particular with respect to terminations and ingoing and outgoing leads. Furthermore, the" working distance that can be obtained is not as small as would be preferred.

Another way of obtaining very small probes is by use of a so-called single-field condenser objective (Rie'cke and Ruska, Proc. 6th Int. Cong. for Electron Microscopy, Kyoto, 1966). This is a lens in which the specimen is situated at, or near to, the lens centre so that only about half the field is used for imaging the elec trons leaving the specimen. (Thus the lens excitation is in general, higher than that of the conventional objective lens). The other half of the lens field, on the source side of the specimen, acts on the illuminating electrons and focuses them onto the specimen. Thus the final SUMMARY OF THE lNVENTlON Briefly the present invention provides an improved electron microscope with a combined condenser and objective lens in which the condenser and objective lenses are separately provided with energising coils and in which the combined lens includes an apertured plate which is clamped so as to separate the condenser and objective lens.

It will be seen that the condenser lens portion of the combined lens can thus be used with a very short working distance, since the condenser and objective lenses are separated only by the thickness of the apertured plate. Thus, it will be appreciated that the apertured plate should preferably be made as thin as possible. However, there is a limit to how thin this plate can be made, since if it is made too thin magnetic saturation will tend to occur within the plate.

This difficulty is overcome in a preferred form of the invention by providing a power supply for energising the magnetic coils, the power supply being so arranged that the magnetic fluxes induced by the coils fiow in opposite senses through the apertured plate, thereby tending to cancel each other out within the plate. This reduces the net magnetic flux within the plate and therefore permits the plate to be thinner than it could otherwise have been.

BRIEF DESCRIPTION OF THE DRAWINGS One electron microscope in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, of which:

FIG. 1 is a schematic sectional elevation of the microscope, including a combined condenser and objective lens, which is shown in somewhat simplified form in this Figure;

FIG. 2 is a sectional elevation of the combined condenser and objective lens in greater detail; and

FIG. 3 is a sectional elevation illustrating a modified form of the combined lens of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, the microscope basically comprises an elongated evacuable column 10, constructed from a number of lens elements 11, 12 and 13 and a number of tubular spacing members 14, clamped together by suitable means. The axis of the column is indicated in the drawing by the reference numeral 15.

An electron gun 16 is positioned at one end of the column, and comprises a V-shaped tungsten filament 17 surrounded by a cup-shaped anode 18. In operation, electrons are emitted thermionically from the filament 17 and are accelerated along the direction of the axis by virtue of a positive voltage applied to the anode with respect to the filament.

The electrons from the gun 16 are focused by means of the lens element 11, which is referred to as the first condenser lens. This lens 11 is an electromagnetic electron optical lens, and since such lenses are well known in the art, it will not be described in detail herein. Briefly, however, this lens comprises a magnetic iron structure 19 which defines a gap 20, and contains a magnetic coil 21. When the coil 21 is energised, a magnetic flux is produced within the iron 19 and across the gap 20. It is this magnetic field within the gap which has the focusing effect on the electrons. in operation,

the lens 11 acts to form a demagnified electron image of the emitting tip of the tungsten filament 17, the position of this image being indicated by the reference numeral 22 in the drawing.

Electrons from the first condenser lens 11 pass into the lens element 12, which is a combined second condenser lens and objective lens for the microscope. The combined lens 12 comprises two tubular iron pole pieces 23 and 24, the bores of which are aligned with the axis 15. Between these pole pieces is positioned an iron plate 25, having an aperture 26 formed therein, this aperture also being aligned with the axis 15. The two pole pieces 23 and 24 and the apertured plate 25 thus define two gaps, which respectively serve as the second condenser lens gap 27 and the objective lens gap 28. The pole pieces 23, 24 and the plate 25 are magnetically coupled by means of tubular iron members 29, 30, formed integrally with the respective pole pieces and providing a magnetic yoke for the combined lens. Separate magnetic coils 31 and 32 are positioned within the members 29, 30, and it will be seen that energisation of these coils 31 and 32 respectively produce magnetic fields in the second condenser lens gap 27 and the objective lens gap 28. Power supply means for the magnetic coils 31, 32 is shown diagrammatically at 40 and may be for example a switched controllable d.c. supply. The magnetic coils 31, 32 are connected to the power supply 40 by respective pairs of conductors 41, 42. A specimen holder 33 is provided for supporting a specimen for examination in the microscope in the objective lens gap 28. Vacuum pumping means is shown diagrammatically at 43 which may be any suitable commercial vacuum pump such as an oil diffusion pump backed by a rotary pump.

In operation of the microscope, the energisation current in the second condenser lens coil 31 is set at a value such that the magnetic field in the second condenser lens gap 27 focuses the electrons from the first condenser lens 11, so as to produce a greatly demagnified electron image of the image 22, on the surface of the specimen in the specimen holder 33. The large demagnification of the second condenser lens is possible because of the very short working distance of this lens, which in turn is due to the fact that the second condenser lens and the objective lens effectively share a common pole piece, i.e., the plate 25, and are therefore separated only by the thickness of that plate. In addition, as previously mentioned, the short working distance reduces the spherical aberration in the second condenser lens.

Thus, it will be seen that the second condenser lens forms a very fine probe for illuminating the specimen. Typically, the size of this probe is of the order of 0.1 micron.

The energisation current in the objective lens coil 32 is set at a value such that the magnetic field within the objective lens gap 28 focuses those electrons which pass through the specimen to form a magnified intermediate image 34 of the area of the specimen which is illuminated by the probe. The lens 13 acts as a projector lens for further magnifying this image 34 to produce a final image on a fluorescent viewing screen 35 at the lower end of the microscope column. The projector lens 13 will not be described in detail herein, since such lenses are well known in the art. It should be appreciated. however, that in other embodiments of the invention, a plurality of projector lens may be employed, instead of one as shown in the drawing.

As mentioned above, the structure of the combined condenser and objective lens 12 is shown in somewhat simplified form in FIG. 1. Referring now to FIG. 2, this shows the combined lens 12 in greater detail and on an enlarged scale.

As will be seen from FIG. 2, the plate 25 is not flat as shown in FIG. 1, but is dished so as to leave more room for a mechanism 36 for positioning the specimen support 33.

The thickness of the plate 25 must be sufficient to carry the magnetic flux entering this plate from the rest of the iron circuit (constituted by the pole pieces 23, 24 and the magnetic yoke 29, 30) without the material of the plate 25 being taken into magnetic saturation. The magnetic flux lines from the pole pieces 23, 24 to the plate 25 enter the plate over all its surface so that the flux within the plate increases in the radially outward direction with increasing distance from the axis 15. Thus, the minimum thickness of the plate depends on the distance from the lens axis. In practice, however, a uniform thickness may be used. A reduction in the thickness of the plate 25, and hence of the working distance of the second condenser lens, is achieved by arranging for the excitation currents in the second condenser and objective coils 29 and 30 to be in such senses that the magnetic fluxes in the gaps 27 and 28 are in the same direction. In this situation, the fluxes in the plate 25 due to these two coils will be in opposite senses, and will therefore tend to cancel each other out, thereby resulting in a very small resultant flux in the plate. By suitable choice of the geometries of the gaps 2'7, 28 it is possible to balance these fluxes almost exactly so that the resulting flux is almost zero. It is not possible, however, to achieve an exact balance under all working conditions because it is required that the currents in the two coils should be independently adjustable. Nevertheless, a substantial reduction in magnetic flux, and hence in thickness of the plate 25, can still be achieved by this means.

Referring still to FIG. 2, four pairs of beam deflector coils (only two pairs visible in the drawing) 37 are positioned within the bore of the pole piece 23. These coils permit the electron beam to be displaced so as to permit adjustment of the beam lateral position and direction at the specimen. The beam deflector coils can also be used to scan the probe over the specimen in a raster pattern, so as to permit the microscope to be used in a scanning mode.

The combined lens also includes an X-ray detector 38 positioned between the plate 25 and the condenser coil 31, for detecting X-rays emitted from the illuminated spot of the specimen, thereby enabling an X-ray analysis of the specimen to be performed. Placing the X-ray detector in this position ensures that it is close to the specimen as is required for good X-ray detection sensitivity and has the further advantage that the magnetic field in the gap 27 through which the X-rays pass to the detector 38 acts as a filter against secondary electrons emitted from the specimen, thus preventing these electrons from reaching the detector and giving rise to spurious signals.

In the embodiment illustrated in FIG. 2, the X rays emerge through the aperture 26 in the plate 25,and hence this aperture must be made large enough to give a direct line of sight from the specimen to the detector 28. This has the disadvantage of allowing the fields in the gaps 27 and 28 to overlap to come extent, thus upsetting the independence of the focusing of the second condenser and objective lenses. Provided, however, the condenser portion of the field does not penetrate the gap 28 significantly beyond the specimen, this is not too serious, since the method of operation can be adopted of first adjusting the current in the coil 32 so as to focus the image, allowing the focusing of the probe to be slightly affected, and then adjusting the current in the coil 31 so as to focus'the probe, this latter adjustment having little effect on the focusing of the image.

Referring now to FIG. 3, in a modification of the combined lens shown in FIG. 2, the aperture 26 in the plate 25 is reduced to as small a size as is feasible, so as to minimise overlapping of the fields in the gaps 27, 28. For practical reasons in making this aperture sufficiently round so as to avoid introducing astigmatism, the smallest aperture is typically of the order of 1 millimetre. A suitable thickness for the plate 25 in the region of the aperture 26 would then be of the order of 2 millimetres.

In this case, the aperture 26 is too small to permit X- rays to pass from the specimen to the detector 28. Instead, an auxiliary aperture 39 is bored in the plate 25 in line between the specimenand the X-ray detector 38. It is important to preserve circular symmetry in the region of the aperture 26, in order to avoid introducing astigmatism or similar aberrations, and for this reason at least three such auxiliary apertures 39 are formed, equally angularly spaced about the axis 15. In the preferred embodiment illustrated :in FIG. 3, four such auxiliary apertures are used; one for the extraction of X- rays and the other three to preserve symmetry.

In a further modification, more than one X-ray detector may be used, each receiving X-rays through one or more of the exit holes. Alternatively, an annular X-ray detector could be used in conjunction with a number of exit holes or slots, provided always that the number of such holes or'slots is not such as to reduce the iron cross section to 'such an extent that the iron goes into magnetic saturation.

The electron microscope of FIG. 1 can also be used in a limited-area electron diffraction mode. This mode involves the formation of an electron diffraction pattern from a small area of the specimen, the area being selected by first of all viewing the specimen in the normal imaging mode. The area can be selected by use of an aperture in the plane of the intermediate image 34 of the specimen, for areas down to about 1 micron in diameter, but the aberration of the objective lens generally prevents this method from being used for areas much smaller than this. Alternatively, for small areas, the area can be selected by reducing the area illuminated by the probe. As has been shown, the combined condenser-objective lens 12 enables the probe area to be reduced to down to a few hundred angstroms diameter.

We claim:

1. A combined condenser and objective lens for an electron microscope comprising:

a. first and second pole pieces, said pole pieces defining, respectively, first and] second apertures, said first and second apertures being aligned on a common axis;

,b. a third, relatively thin pole piece disposd intermediate of and spaced from said first and second pole I pieces for defining respective condenser and objective lens gaps between the first, third, and second pole pieces, said third pole piece defining a third aperture aligned with said common axis;

c. a magnetic yoke coupling said first and second pole pieces and said third pole piece;

d. first and second electrical windings connected to said first and second pole pieces, respectively;

"e. power supply means for inducing, in operation of said combined lens, magnetic flux through said magnetic yoke and across said first and second gaps, respectively, and

f. mounting means for mounting a specimen within said second gap.

2. The combined condenser and objective lens of claim 1 wherein, in operation of said combined lens, said first and second windings are connected to said power supply means so that the respective magnetic fluxes induced by said windings in said third pole piece flow in directions such as to tend to cancel.

3. The combined condenser and objective lens of claim 1, further comprising:

an X-ray detector disposed adjacent the side of said third pole piece nearer to said first winding and offset from said common axis, to receive, in operation of the combined lens, X-rays emitted from the specimen in response to electrons focused by the first gap and passing through said third aperture.

4. The combined condenser and objective lens of claim 1 wherein said third pole piece is a plate which defines a plurality of additional apertures which are arranged symmetrically around said common axis.

5. The combined condenser and objective lens of claim 1 wherein at least one further winding is connected to said first pole piece, said further winding producing upon actuation, lateral deflecting magnetic flux.

6. The combined condenser and objective lens of claim 1 wherein said third pole piece has a dished configuration.

7. An electron microscope comprising:

' a. a specimen holder;

b. a housing defining a chamber in which said specimen holder is located;

c. pumping means for evacuating said chamber;

d. an electron gun for directing an electron beam toward said specimen, and

e. a combined condenser and objective lens, comprising:

i. first and second pole pieces, said pole pieces defining, respectively, first and second apertures, said first and second apertures being aligned on a common axis;

ii. a relatively thin magnetic plate positioned between said first and second pole pieces, said plate defining a third aperture, said third aperture being also aligned on said common axis, said first and second pole pieces and said plate thereby defining first and second gaps between said first and second pole pieces and said plate, respectively, said first and second gaps serving as condenser and objective lens gaps, respectively;

iii. a magnetic yoke coupling said first and second pole pieces and said plate;

iv. first and second electrical windings embracing said first and second pole pieces, respectively;

v. a power supply for inducing, in operation of said combined lens, magnetic flux through said magnetic yoke and across said first and second gaps, respectively, and

vi. mounting structure for mounting a specimen within said second gap so that said specimen may be illuminated by the electron beam from said electron gun.

8. The electron microscope of claim 7, further comprising;

said power supply being connected to said first and second windings to actuate said first and second windings to produce magnetic flux in said plate in directions having opposing components.

9. A combined condenser and objective lens for an electron microscope comprising:

a. first and second pole pieces, said pole pieces defining, respectively, first and second apertures, said first and second apertures being aligned on a common axis;

b. a plate positioned between said first and second pole pieces, said plate defining a third aperture, said third aperture being also aligned on said common axis, said first and second pole pieces and said plate thereby defining respective first and second gaps between said first and second pole pieces and said plate, respectively, said first and second gaps serving as condenser and objective lens gaps, respectively;

c. electrical windings connected to said first and second pole pieces, said windings being connectable to an electric power supply for causing said pole pieces to produce magnetic flux across said first and second gaps; and,

d. wherein the thickness of said plate is selected as substantially the minimum thickness for preventing magnetic saturation of the plate for the level of flux produced within the plate during operation of the microscope.

10. The lens of claim 9, wherein:

said windings and said power supply are electrically connected to cause said magnetic flux across said first and second gaps, respectively, to have directional components extending in opposite directions.

11. A combined condenser and objective lens for an electron microscope comprising:

a. first and second pole pieces defining, respectively, first and second apertures which are aligned along a common axis;

b. a third, relatively thin pole piece disposed intermediate of and spaced from said first and second pole pieces for defining respective condenser and objective lens gaps between the first, third, and second pole pieces, said third pole piece defining a third aperture aligned with said common axis;

c. magnetic means coupling said first, second and third pole pieces;

d. means for inducing magnetic flux through said coupling means, said pole pieces and across said gaps for electron focusing; and,

e. mounting means for mounting a specimen within said second gap.

12. The lens according to claim 11 wherein the thickness of said third pole piece is selected to provide minimum values for said gaps and is further selected in accordance with said inducing means for assuring that the third pole piece does not become magnetically saturated during operation.

13. An electron microscope comprising: a. a specimen holder; b. a housing defining a chamber in which said specimen holder is located; c. pumping means for evacuating said chamber; d. an electron gun for directing an electron beam toward said specimen; e. a combined condenser and objective lens, comprising:

i. first and second pole pieces, said pole pieces defining, respectively, first and second apertures, said first and second apertures being aligned on a common axis;

ii. a plate positioned between said first and second pole pieces, said plate defining a third aperture aligned on said common axis and a plurality of additional apertures arranged symmetrically around the common axis, said first and second pole pieces and said plate thereby defining first and second gaps between said first and second pole pieces and said plate, respectively, said first and second gaps serving as condenser and objective lens gaps, respectively;

iii. a magnetic yoke coupling said first and second pole pieces and said plate;

iv. first and second electrical windings connected to said first and second pole pieces, respectively;

v. a power supply for inducing, in operation of said combined lens, magnetic flux through said magnetic yoke and across said first and second gaps, respectively;

vi. mounting structure for mounting a specimen within said second gap so that said specimen may be illuminated by the electron beam from said electron gun; and,

. an X-ray detector disposed adjacent the side of said said specimen in response to impinging electrons. 

1. A combined condenser and objective lens for an electron microscope comprising: a. first and second pole pieces, said pole pieces defining, respectively, first and second apertures, said first and second apertures being aligned on a common axis; b. a third, relatively thin pole piece disposd intermediate of and spaced from said first and second pole pieces for defining respective condenser and objective lens gaps between the first, third, and second pole pieces, said third pole piece defining a third aperture aligned with said common axis; c. a magnetic yoke coupling said first and second pole pieces and said third pole piece; d. first and second electrical windings connected to said first and second pole pieces, respectively; e. power supply means for inducing, in operation of said combined lens, magnetic flux through said magnetic yoke and across said first and second gaps, respectively, and f. mounting means for mounting a specimen within said second gap.
 2. The combined condenser and objective lens of claim 1 wherein, in operation of said combined lens, said first and second windings are connected to said power supply means so that the respective magnetic fluxes induced by said windings in said third pole piece flow in directions such as to tend to cancel.
 3. The combined condenser and objective lens of claim 1, further comprising: an X-ray detector disposed adjacent the side of said third pole piece nearer to said first winding and offset from said common axis, to receive, in operation of the combined lens, X-rays emitted from the specimen in response to electrons focused by the first gap and passing through said third aperture.
 4. The combined condenser and objective lens of claim 1 wherein said third pole piece is a plate which defines a plurality of additional apertures which are arranged symmetrically around said common axis.
 5. The combined condenser and objective lens of claim 1 wherein at least one further winding is connected to said first pole piece, said further winding producing upon actuation, lateral deflecting magnetic flux.
 6. The combined condenser and objective lens of claim 1 wherein said third pole piece has a dished configuration.
 7. An electron microscope comprising: a. a specimen holder; b. a housing defining a chamber in which said specimen holder is located; c. pumping means for evacuating said chamber; d. an electron gun for directing an electron beam toward said specimen, and e. a combined condenser and objective lens, comprising: i. first and second pole pieces, said pole pieces defining, respectively, first and second apertures, said first and second apertures being aligned on a common axis; ii. a relatively thin magnetic plate positioned between said first and second pole pieces, said plate defining a third aperture, said thIrd aperture being also aligned on said common axis, said first and second pole pieces and said plate thereby defining first and second gaps between said first and second pole pieces and said plate, respectively, said first and second gaps serving as condenser and objective lens gaps, respectively; iii. a magnetic yoke coupling said first and second pole pieces and said plate; iv. first and second electrical windings embracing said first and second pole pieces, respectively; v. a power supply for inducing, in operation of said combined lens, magnetic flux through said magnetic yoke and across said first and second gaps, respectively, and vi. mounting structure for mounting a specimen within said second gap so that said specimen may be illuminated by the electron beam from said electron gun.
 8. The electron microscope of claim 7, further comprising; said power supply being connected to said first and second windings to actuate said first and second windings to produce magnetic flux in said plate in directions having opposing components.
 9. A combined condenser and objective lens for an electron microscope comprising: a. first and second pole pieces, said pole pieces defining, respectively, first and second apertures, said first and second apertures being aligned on a common axis; b. a plate positioned between said first and second pole pieces, said plate defining a third aperture, said third aperture being also aligned on said common axis, said first and second pole pieces and said plate thereby defining respective first and second gaps between said first and second pole pieces and said plate, respectively, said first and second gaps serving as condenser and objective lens gaps, respectively; c. electrical windings connected to said first and second pole pieces, said windings being connectable to an electric power supply for causing said pole pieces to produce magnetic flux across said first and second gaps; and, d. wherein the thickness of said plate is selected as substantially the minimum thickness for preventing magnetic saturation of the plate for the level of flux produced within the plate during operation of the microscope.
 10. The lens of claim 9, wherein: said windings and said power supply are electrically connected to cause said magnetic flux across said first and second gaps, respectively, to have directional components extending in opposite directions.
 11. A combined condenser and objective lens for an electron microscope comprising: a. first and second pole pieces defining, respectively, first and second apertures which are aligned along a common axis; b. a third, relatively thin pole piece disposed intermediate of and spaced from said first and second pole pieces for defining respective condenser and objective lens gaps between the first, third, and second pole pieces, said third pole piece defining a third aperture aligned with said common axis; c. magnetic means coupling said first, second and third pole pieces; d. means for inducing magnetic flux through said coupling means, said pole pieces and across said gaps for electron focusing; and, e. mounting means for mounting a specimen within said second gap.
 12. The lens according to claim 11 wherein the thickness of said third pole piece is selected to provide minimum values for said gaps and is further selected in accordance with said inducing means for assuring that the third pole piece does not become magnetically saturated during operation.
 13. An electron microscope comprising: a. a specimen holder; b. a housing defining a chamber in which said specimen holder is located; c. pumping means for evacuating said chamber; d. an electron gun for directing an electron beam toward said specimen; e. a combined condenser and objective lens, comprising: i. first and second pole pieces, said pole pieces defining, respectively, first and second apertures, said first and second aperturEs being aligned on a common axis; ii. a plate positioned between said first and second pole pieces, said plate defining a third aperture aligned on said common axis and a plurality of additional apertures arranged symmetrically around the common axis, said first and second pole pieces and said plate thereby defining first and second gaps between said first and second pole pieces and said plate, respectively, said first and second gaps serving as condenser and objective lens gaps, respectively; iii. a magnetic yoke coupling said first and second pole pieces and said plate; iv. first and second electrical windings connected to said first and second pole pieces, respectively; v. a power supply for inducing, in operation of said combined lens, magnetic flux through said magnetic yoke and across said first and second gaps, respectively; vi. mounting structure for mounting a specimen within said second gap so that said specimen may be illuminated by the electron beam from said electron gun; and, f. an X-ray detector disposed adjacent the side of said plate nearer the first windings and offset from said common axis for detecting X-rays emitted from said specimen in response to impinging electrons. 